K2Pb(H2C3N3O3)4(H2O)4: a potential UV nonlinear optical material with large birefringence

Cyanurate motifs are emerging functional basic blocks (FBBs) for nonlinear optical (NLO) materials and birefringent materials. Herein, we report two new potassium lead(II) hydroisocyanurates, namely, K2Pb(H2C3N3O3)4(H2O)4 (I) and K3Pb(H2C3N3O3)5(H2O)6 (II). I and II belong to acentric Cm and centric C2/m, respectively. Compound I features a novel 3D network formed by 2D [K2PbO8(H2O)4]12− anionic layers interconnected by (H2C3N3O3)− anions. The 2D [K2PbO8(H2O)4]12− anionic layer is composed of 1D chains of edge-sharing K(1)O6 and K(2)O8 being further interconnected by 8-coordinated Pb2+ ions. Compound II exhibits a characteristic 3D framework based on 2D [K3PbO11(H2O)6]17− anionic layers interlinked through (H2C3N3O3)− groups, and the 2D [K3PbO11(H2O)6]17− anionic layer is based on K–O/H2O chains bridged by Pb2+ ions. Powder second harmonic generation (SHG) measurements of I revealed a moderate SHG response of approximately 2.6 × KDP with a type I phase-matching behavior under 1064 nm laser radiation. Theoretical calculations of I and II revealed their birefringence to be 0.325 and 0.193@532 nm, respectively. Their impressive optical properties mainly stem from stereochemically active lone pair (SCALP) Pb2+ cations and the planar π-conjugated (H2C3N3O3)− groups.